In the use of apparatus employing a hydraulic fluid, the performance of the apparatus can be affected by a change in the dielectric properties of the hydraulic fluid. The change may be the result of lot to lot differences in the hydraulic fluid and it may result from air or other material being absorbed into the hydraulic fluid. Frequently control of dielectric property changes is made more difficult by the fact that there may be more than one parameter producing the change within a single range.
The problem is particularly acute in hydraulic cylinder piston position sensing systems that handle the increasingly stringent operational criteria encountered in precisely, reliably and continuously sensing the position of the piston and it's related displacement parameters, velocity and acceleration using an electromagnetic signal. A correlation is made of resonances of electromagnetic waves in the cavity that is made up of the cylinder and the piston, with the dimensions of the cavity, where the wave performance is similar to that of an electrical transmission line with shorted ends, in that the resonance frequency of the electromagnetic standing wave correlates inversely with the cavity length.
In U.S. Pat. No. 4,737,705, a coaxial resonant cavity with a central core is employed within which the electromagnetic waves are launched and propagated in the mode referred to as the transverse electromagnetic wave (TEM) mode. The cylinder on the rod side of the piston is one type of coaxial cavity.
In U.S. Pat. No. 5,182,979, compensation, through equalization, is provided for differences in insertion losses as the linear extension of the piston and rod in the cylinder takes place.
In U.S. Pat. No. 5,325,063 identification of a fundamental resonance frequency is enhanced by the use of positive and negative slope intercepts of a resonance signal with respect to a reference level, where the positive slope is at the lower frequency and the negative slope is at the higher frequency.
As progress in the art of electromagnetic wave piston position sensing in hydraulic cylinders continues, the effects of the condition of the hydraulic fluid resulting from wide pressure and temperature range operation and contamination such as that existing in lot to lot oil supplies and from air and other contaminant entrainment through leaking seals, may introduce error and interfere with achieving desired precision.